In industrial measurements technology, especially also in connection with the control and monitoring of automated manufacturing processes, for ascertaining the characteristic measured variables of a media, for example, liquids and/or gases, flowing in a process line, for example, a pipeline, measuring systems are often used; which induce, by means of a measuring transducer of the vibration-type and a driver, and evaluating, electronics connected thereto and most often accommodated in a separate electronics housing, reaction forces, for example, Coriolis forces, in the flowing medium. Derived from these reaction forces, a measurement signal correspondingly representing the at least one measured variable, for example, a mass flow, a density, a viscosity or another process parameter is produced.
Measuring systems of this kind, which are often formed by means of an inline measuring device in compact construction with an integrated measuring transducer, such as, for instance, a Coriolis mass flow meter, have been known for a long time and have proven themselves in industrial use. Examples of such measuring systems having a measuring transducer of the vibration-type, or also individual components thereof, are described e.g. in EP-A 317 340, U.S. Pat. No. 4,738,144, U.S. Pat. No. 4,777,833, U.S. Pat. No. 4,823,614, U.S. Pat. No. 5,287,754, U.S. Pat. No. 5,291,792, U.S. Pat. No. 5,301,557, U.S. Pat. No. 5,398,554, U.S. Pat. No. 5,476,013, U.S. Pat. No. 5,531,126, U.S. Pat. No. 5,602,345, U.S. Pat. No. 5,610,342, U.S. Pat. No. 5,731,527, U.S. Pat. No. 5,691,485, U.S. Pat. No. 5,796,010, U.S. Pat. No. 5,796,012, U.S. Pat. No. 5,796,011, U.S. Pat. No. 5,945,609, U.S. Pat. No. 5,979,246, U.S. Pat. No. 6,047,457, U.S. Pat. No. 6,092,429, U.S. Pat. No. 6,168,069, U.S. Pat. No. 6,223,605, U.S. Pat. No. 6,311,136, U.S. Pat. No. 6,330,832, U.S. Pat. No. 6,397,685, U.S. Pat. No. 6,557,422, U.S. Pat. No. 6,519,828, U.S. Pat. No. 6,666,098, U.S. Pat. No. 6,378,364, U.S. Pat. No. 6,691,583, U.S. Pat. No. 6,840,109, U.S. Pat. No. 6,860,158, U.S. Pat. No. 6,883,387, U.S. Pat. No. 6,651,513, U.S. Pat. No. 6,758,102, U.S. Pat. No. 6,920,798, U.S. Pat. No. 7,080,564, U.S. Pat. No. 7,073,396, U.S. Pat. No. 7,077,014, U.S. Pat. No. 7,040,179, U.S. Pat. No. 7,017,424, U.S. Pat. No. 7,213,469, U.S. Pat. No. 7,299,699, U.S. Pat. No. 7,337,676, U.S. Pat. No. 7,340,964, U.S. Pat. No. 7,360,451, U.S. Pat. No. 7,392,709, US-A 2006/0201260, US-A 2007/0186685, US-A 2007/0151371, US-A 2007/0151370, US-A 2007/0119265, US-A 2007/0119264, US-A 2008/0141789, US-A 2008/0047361, the JP-A 8-136311, the JP-A 9-015015, WO-A 08/059015, WO-A 08/013545, WO-A 01 02 816, WO-A 00 14 485 or WO-A 99 40 394. Each of the therein illustrated, measuring transducers comprises at least one, essentially straight, or at least one, curved, measuring tube for conveying the medium, which can, in given cases, also be extremely cold or extremely hot.
In the operation of the measuring system, the at least one measuring tube is caused to vibrate during operation for the purpose of generating oscillation forms influenced also by the medium flowing through the measuring tube.
For exciting oscillations of the at least one measuring tube, measuring transducers of the vibration-type include, additionally, an exciter mechanism driven during operation by an electrical driver signal e.g. in the form of a controlled electrical current, generated and correspondingly conditioned by the mentioned driver electronics. The exciter mechanism excites the measuring tube during operation by means of at least one electromechanical, especially electrodynamic, oscillation exciter, through which an electrical current flows and which acts essentially directly on the measuring tube, such that the measuring tube executes bending oscillations in the wanted mode. Furthermore, such a measuring transducer includes a sensor arrangement having oscillation sensors, especially electrodynamic oscillation sensors, for at least pointwise registering of inlet side and outlet side oscillations of the at least one measuring tube, especially those in the Coriolis mode, and for producing electrical sensor signals influenced by the process parameter to be registered, such as, for instance, the mass flow or the density.
Selected as excited oscillation form, the so-called wanted mode, in the case of measuring transducers with a curved measuring tube, e.g. U, V or Ω shaped, is usually that of an eigenoscillation form, in the case of which the measuring tube moves like a pendulum at least partially in a lowest natural resonance frequency about an imaginary longitudinal axis of the measuring transducer in the manner of a cantilever clamped at one end, whereby Coriolis forces are induced in the medium flowing through the measuring tube as a function of the mass flow. This, in turn, leads to the fact that, superimposed on the excited oscillations of the wanted mode, in the case of curved measuring tubes, thus, pendulum-like, cantilever oscillations, are bending oscillations of equal frequency according to at least one, likewise natural, second oscillation form, the so-called Coriolis mode. In the case of measuring transducers with curved measuring tubes, these cantilever oscillations in the Coriolis mode brought about by Coriolis forces correspond usually to the eigenoscillation form, in which the measuring tube also executes rotary oscillations about an axis perpendicular to the longitudinal axis. In the case of measuring transducers with straight measuring tubes, in contrast, for the purpose of producing Coriolis forces dependent on mass flow, often such a wanted mode is selected, wherein the measuring tube executes, at least partially, bending oscillations essentially in a single plane of oscillation, so that the oscillations in the Coriolis mode are formed accordingly as bending oscillations of equal oscillation frequency coplanar with the oscillations of the wanted mode.
Due to the superpositioning of the wanted mode and Coriolis mode, the oscillations of the vibrating measuring tube registered by means of the sensor arrangement on the inlet side and on the outlet side have a measurable phase difference also dependent on mass flow. Usually, the measuring tubes of such measuring transducers applied e.g. in Coriolis mass flow meters are excited during operation to an instantaneous, natural, resonance frequency of the oscillation form selected for the wanted mode, especially an oscillation form having an oscillation amplitude controlled to be constant. Since this resonance frequency depends, especially, also on the instantaneous density of the medium, usually marketed Coriolis mass flow meters can measure, besides the mass flow, supplementally also the density of flowing media. Additionally, it is also possible, such as, for example, shown in U.S. Pat. No. 6,651,513 or U.S. Pat. No. 7,080,564, directly to measure by means of measuring transducers of the vibration-type also the viscosity of the medium flowing through the measuring tube, for example, based on an excitation power required for exciting the oscillations.
In the case of measuring transducers having two measuring tubes, these are most often integrated into the process line via a distributor piece extending on the inlet side between the measuring tubes and an inlet side, connecting flange, as well as via a distributor piece extending on the outlet side between the measuring tubes and an outlet side connecting flange. In the case of measuring transducers with a single measuring tube, such communicates with the process line most often via an essentially straight, connecting tube piece on the inlet side as well as via an essentially straight, connecting tube piece on the outlet side. Additionally, each of the disclosed measuring transducers having a single measuring tube includes at least one counteroscillator embodied as one piece or constructed from a plurality of parts, for example, a tube-, box- or plate-shaped counteroscillator, which is coupled to the measuring tube on the inlet side to form a first coupling zone and on the outlet side to form a second coupling zone, and which, during operation, essentially either rests or else oscillates opposite-equally to the measuring tube, thus with equal frequency and opposite phase. The inner part of the measuring transducer formed by means of measuring tube and counteroscillator is most often held only by means of the two connecting tube pieces, via which the measuring tube communicates with the process line during operation, in a protective measuring transducer housing, especially in a manner enabling oscillations of the inner part relative to the measuring tube. In the case of the measuring transducers, illustrated, for example, in U.S. Pat. No. 5,291,792, U.S. Pat. No. 5,796,010, U.S. Pat. No. 5,945,609, U.S. Pat. No. 7,077,014, US-A 2007/0119264, WO-A 01/02 816 or also WO-A 99/40 394, having a single, essentially straight, measuring tube, the latter and the counteroscillator are, such as in the case of conventional measuring transducers quite usual, oriented essentially coaxially relative to one another. In the case of the usually marketed measuring transducers of the aforementioned type, most often, counteroscillator is also embodied essentially tubularly in the form of an essentially straight, hollow cylinder, which is so arranged in the measuring transducer, that the measuring tube is jacketed, at least partially, by the counteroscillator. Used as materials for such counteroscillators, especially also in the case of the application of titanium, tantalum or zirconium for the measuring tube, are, most often, comparatively cost-effective steel types, such as, for instance, structural steel or free-machining steel.
The exciter mechanism of measuring transducers of the type being discussed includes, usually, at least one electrodynamic oscillation exciter and/or an oscillation exciter, which acts differentially on the at least one measuring tube and the, in given cases, present counteroscillator or the, in given cases, present, other measuring tube, while the sensor arrangement includes an inlet side, most often likewise electrodynamic, oscillation sensor as well as at least one, essentially equally-constructed, outlet side oscillation sensor. Such electrodynamic and/or differential oscillation exciters of usually marketed measuring transducers of vibration-type are formed by means of a magnet coil, through which an electrical current flows, at least at times, and which, in the case of measuring transducers having a measuring tube and a counteroscillator coupled thereto, is most often affixed to the latter, as well as by means of a permanent magnet interacting with the at least one magnet coil, especially a permanent magnet plunging into the coil and serving as an armature of rather elongated, especially rod, shape, affixed correspondingly to the measuring tube to be moved. The permanent magnet and the magnet coil serving as an exciter coil are, in such case, usually so oriented, that they extend essentially coaxially relative to one another. Additionally, in the case of conventional measuring transducers, the exciter mechanism is usually embodied and placed in the measuring transducer in such a manner, that it acts essentially centrally on the at least one measuring tube. In such case, the oscillation exciter and, insofar, the exciter mechanism, is, such as, for example, also shown in the case of the measuring transducers disclosed in U.S. Pat. No. 5,796,010, U.S. Pat. No. 6,840,109, U.S. Pat. No. 7,077,014 or U.S. Pat. No. 7,017,424, most often affixed to the measuring tube at least pointwise along an imaginary central, circumferential line on the outside of the measuring tube. Alternatively to an exciter mechanism formed by means of oscillation exciters acting rather centrally and directly on the measuring tube, it is also possible to use, such as disclosed in, among others, U.S. Pat. No. 6,557,422, U.S. Pat. No. 6,092,429 or U.S. Pat. No. 4,823,614, for example, also exciter mechanisms formed by means of two oscillation exciters affixed to the measuring tube not in the center of the measuring tube, but, instead rather at the inlet and outlet sides thereof or, such as disclosed in, among others, U.S. Pat. No. 6,223,605 or U.S. Pat. No. 5,531,126, for example, also by means of an oscillation exciter acting between the, in given cases, present counteroscillator and the measuring transducer housing.
In the case of most, usually marketed, measuring transducers of the vibration-type, the oscillation sensors of the sensor arrangement are, as already indicated, at least essentially of construction equal to that of the at least one oscillation exciter, since they work according to the same principle of action. Accordingly, also the oscillation sensors of such a sensor arrangement are, most often, formed, in each case, by means of at least one magnet coil, which is usually affixed to the, in given cases, present counteroscillator, at least at times passed through by a variable magnetic field and, associated therewith, at least at times supplied with an induced measurement voltage, as well as by means of a rod-shaped permanent magnet affixed to the measuring tube, interacting with the at least one magnet coil and delivering the magnetic field. Each of the aforementioned coils is additionally connected by means of at least one pair of electrical connecting lines with the mentioned operating and evaluating electronics of the inline measuring device. These electrical connecting lines are led, most often, on shortest possible paths from the coils via the counteroscillator to the transducer housing.
In the case of measuring transducers of the type being discussed, as mentioned, in among others, also in U.S. Pat. No. 6,047,457 or U.S. Pat. No. 6,920,798, it is additionally usual to secure magnet coil and the therewith corresponding permanent magnets of the oscillation exciter or the oscillation sensors to ring- or washer-shaped, especially metal, securement elements mounted on the measuring tube and fixedly encircling the measuring tube, in each case, essentially along one of its imaginary, circumferential lines. The particular securement element can, as disclosed in among others, U.S. Pat. No. 6,047,457, U.S. Pat. No. 7,299,699, US-A 2006/0201260, U.S. Pat. No. 5,610,342, or U.S. Pat. No. 6,519,828, be affixed by pressing from the outside, by hydraulic pressing or rolling from within the measuring tube or by thermal shrink-fitting to the measuring tube, especially in such a manner, that it is lastingly subjected to elastic or mixed plastic-elastic deformations and, as a result of this, is permanently radially prestressed against the measuring tube.
As discussed in, among others, the initially cited US-A 2008/0141789, U.S. Pat. No. 7,318,356, U.S. Pat. No. 6,920,798, U.S. Pat. No. 6,868,740, U.S. Pat. No. 6,758,102, U.S. Pat. No. 5,734,112, U.S. Pat. No. 5,731,527, U.S. Pat. No. 5,576,500 or U.S. Pat. No. 5,301,557, measuring transducers of the vibration-type and, insofar, the entire measuring system formed therewith, can have, besides the initially mentioned sensitivity to the primary measured variables, mass flow or density and, in given cases, also viscosity, also a certain cross-sensitivity to pressure, this being, especially, the case, when the medium has two or more phases, for instance as a liquid gas mixture. This pressure sensitivity can possibly lead to a, though slight, nevertheless, because of the desired high accuracy of measurement, not, without more, disregardable corruption of the primary measured value, such as, for instance, the mass flow, or can lead to measures being required for correspondingly compensating the measuring errors.
An opportunity for counteracting the undesired cross-sensitivity of such measuring transducers to pressure can, such as proposed e.g. in U.S. Pat. No. 6,920,798, involve use of metal rings or similar metal bodies, which coaxially encompass the measuring tube in regions especially critical for pressure sensitivity, such as, for instance, possibly present transitions between straight and curved tube segments. As additionally brought out in U.S. Pat. No. 6,920,798, such a metal body can simultaneously also serve as a securement element of the aforementioned type.
Further investigations on such measuring systems have, however, shown, that, besides the regions of measuring transducers of the type being discussed already identified as especially critical for pressure sensitivity, still other disturbance sources provoking pressure dependencies within such measuring transducers are to be cared for, which necessitate other measures in such measuring transducers for reducing undesired cross-sensitivity to pressure. Especially, in such case, it has been determined, that fast changes of pressure in the flowing medium, such as, for instance, pressure surges caused by valves and/or pulsating pressure fluctuations caused by pumps, can have considerable influence on the required extremely high accuracy of measurement of measuring systems of the type being discussed. Identified as especially critical have been, furthermore, stroke movements of the oscillation sensors associated with pressure surges in the measuring tube, which can occur with, for the accuracy of measurement, not disregardable amplitudes in the radial direction of the measuring tube, especially, also, in spite of application of the ring- or washer-shaped metal bodies of U.S. Pat. No. 6,047,457 as securement elements.